The invention relates to a process for forming a high-strength, wear-resistant composite layer on the surface of an aluminum alloy substrate.
For components made from Al—Si alloys, it is preferable to use hypereutectic alloys, since such alloys have proven particularly advantageous with regard to wear and minimization of friction. To obtain a sufficient number and size of the primary silicon crystals, the aluminum alloys contain, for example, 14 to 17% of silicon. In addition to aluminum, coarse silicon crystals are also formed in the alloy. Etching processes which reduce the thickness of the aluminum cause the wear-resistant, coarse silicon crystals to project, while the recessed aluminum makes it possible to build up a stable lubricating film.
A higher wear resistance in aluminum alloys can already be improved considerably by hardening by modification of the substrate surfaces, for example by partially melting the surface using a laser beam. The result is an increase in strength at the surface.
EP 0 411 322 has disclosed a process which is used to produce wear-resistant surfaces on components made from an Al—Si alloy. For this purpose, the surfaces are coated with a layer comprising a binder, pulverulent silicon, an inoculant for primary silicon crystals and a flux, and then this coating is melted by means of laser energy. The addition of hard materials, for example in the form of metal carbides or metal nitrides, already effects a considerable increase in the surface hardness. One simple method of applying the alloying elements is provided by the screen-printing technique.
Moreover, DE 40 40 436 has disclosed a process for producing wear-resistant layers on cylinder liners made from light metal alloys, in which the entire cylinder liner is subjected to a solid-liquid-solid phase transition by means of high-energy beams—laser or electron beams—and then mechanical remachining is carried out. To increase the surface hardness, the layers may be alloyed with small amounts of iron or nickel and provided with hard materials. The piston surfaces which are to be treated by way of example are in this case first of all electroplated with a selected metal in a first process step.
However, the alloying fractions used in the known processes are restricted to phases which do not achieve a satisfactory hardness. It would be desirable to further increase the resistance of the component surface to wear.